[0001] This invention is directed to a visible light filter in a head up display unit to
filter out solar radiation capable of washing out the display information.
[0002] Typically, in a head up display (hereinafter also referred to as a HUD) ofthetype
exemplified by the description in U.S. Patent No. 3,940,204 issued on February 24,
1976 to R. J. Withrington et al and entitled "Optical Display Systems Utilizing Holographic
Lenses", an observer in a cockpit looks through a holographic diffraction grating
lens at a scene. An image of display information from a source, such as a cathode
ray tube located in the aircraft out of the line of sight, is combined with and superimposed
on the scene by means of an optics system which includes relay optics, a folding reflection
and the holographic lens. In operation, the optics system directs the image of the
display information to the holographic lens where it is diffracted or reflected from
the holograph lens to the viewers eyes whereupon it is effectively combined with the
scene. As a result of this combining function the holographic lens will hereinafter
also be referred to as a combiner.
[0003] Head up displays ofthistype can be subjectedto solar noise in the form of solar radiation
which, when the sun is within a range of certain critical incident angles, will pass
back down the optical path to wash out the display information on the cathode ray
tube.
[0004] A number of possible solutions to this solar noise problem have been investigated,
but most work effectively only over a limited angular range ofincidentangles of incoming
radiation. Examples are narrowband reflection or transmission filters. A typical narrowband
transmission filter is made of a high index of refraction material, typically thin
layers of metals and dielectrics, which provides a narrow bandpass with high transmittance.
This type of filter is preferably tilted across the optical axis in front of the relay
optics with the cathode ray tube being positioned in back of the relay optics. In
operation, this filter reduces the amount of solar radiation which is directed to
the face of the cathode ray tube over a limited range of incoming sun ray angles.
[0005] Another approach is to use a narrowband reflecting mirrorwhich operates somewhat
similarly to the tilted transmission filter but in a reflection rather than a transmission
mode. Often the mirror includes a layer of absorption glass positioned before the
dichroic mirror reflection surface. Again the disadvantage of this approach is the
limited angular range of effectiveness.
[0006] Other solutions which have been investigated but which have not proven successful
are a moving mechanical blocking apparatus and a venetian blind type of blocking element.
[0007] The present invention employs a form of contrast enhancement, the general principle
of which is outlined in the article "Exploitation of Photo- chronic Glass" by G. K.
Megla in Optics and Laser Technology, April 1974 pp 61-68, especially Figure 11.
[0008] According to the present invention, there is provided a visible light filter for
a head up display which includes a display means for producing an image of display
information and an optical system including lens means for directing the image of
the display information to the eye of a viewer, characterized by optical filter means
positioned between the display means and the lens means which is reversibly responsive
to focused visible light directed through the lens means back toward the display means
to block the focused visible light and which is transmissive to the display information
except at the localized portion where focused visible light is blocked.
[0009] The invention will block incoming solar radiation from washing out display information.
It achieves this because solar radiation transmitted back down an optical path toward
the surface of a display device is focused on a small portion of the photochromic
element. This causes the photochromic element to reversibly darken at the point of
focus thereby blocking the solar energy. The remaining area of the solar filter on
which solar radiation is not focused, transmits the lower intensity display information.
[0010] More specifically, in a diffraction head up display the display information on a
cathode ray tube is within a narrow bandwidth of light and its image is focused forward
along an optical path through the solar filter, through a relay lens system and to
a folding mirror from which it is reflected to the combiner lens. The combiner lens
includes a diffraction grating element which reflects, by diffraction, only a narrow
bandwidth of lightwhich includes the wavelengths of the display information. This
diffracted image isthen directed to an exit pupil for viewing.
[0011] When broad spectrum solar radiation shines on the other side of the combiner lens,
the combiner rejects by diffraction that portion of the sunlight which is in the narrow
bandwidth of the display information. But the combiner transmits the remainder of
the broad spectrum solar energy.
[0012] When the sun is within a range of overhead angles, this remainder of the solar energy
could otherwise wash out the display information by passing back down the optical
path to the face of the cathode ray tube. However this wash out is prevented by the
filter of this invention. This filter is placed in the back focal plane of the relay
optics, so that any solar radiation which passes back down the optical path is focused
on a very small spot on the filter. Because the photochromic material of the filter
darken with the high energy density at the point of focusing, the solar energy is
effectively blocked from reaching the cathode ray tube face.
[0013] An advantage to this filter is that it will block solar energy for any angle of the
sun relative to the head up display. As the sun's incident angle continuously changes,
the focused spot correspondingly moves across the solar filter blocking solar energy
at the darkening rate of the photochromic material. When the focused solar energy
moves from a given filter area, the reversible photochromic material lightens again.
[0014] Another advantage of this invention is that the darkened spot within the photochromic
material is small enough so that it will not usually result in a significant blocking
of the display information. For example, a viewer with two eyes will see past the
darkened spot to every display point with at least one eye and thus will not lose
any display information. Furthermore, even if the darkened spot did cover a critical
viewing area for a given eye, the viewer could move his head and eye laterally relative
to the viewing axis to see the display information that had been previously blocked.
[0015] Still another advantage of this invention is that the solar filter element can be
positioned along the optical axis in back of the relay optics without adding to the
optical path length. Thus, the overall dimension of the head up display unit can be
kept short and within the space constraints of an aircraft cockpit.
Brief description of the drawings
[0016] In the accompanying drawings:
Fig. 1 is a schematic, side elevation view of a head up display, optical system illustrating
ray traces for forward focusing of the display information from a cathode ray tube
to the eye of a viewer;
Fig. 2 is the schematic side elevation view of the system of Fig. 1 illustrating ray
traces for the backward focusing of solar radiation toward the display information
on the face of the cathode ray tube;
Fig. 3 is a schematic side elevation view illustrating a preferred embodiment of the
HUD optics system in more detail; and
Fig. 4 is a second embodiment of the solar filter.
Detailed description of the invention
[0017] Referring now to the drawings in more detail, Fig. 1 schematically illustrates the
HUD optical system in which information displayed on an image surface such as the
faceplate of a cathode tube, is focused forward or upstream to an exit pupil which
can be thought of as an area in space. Hereinafter, the term "up" and "forward" will
be used to describe the direction of ray tracing from the cathode ray tube toward
the exit pupil; and the terms "down", and "back" shall be used to describe the opposite
direction of ray tracings toward the cathode ray tube. A viewer, such as a pilot,
positions his eye within this exit pupil so as to view the image from the cathode
ray tube 10 as combined with and superimposed on the scene ahead of the vehicle.
[0018] More specifically, the cathode ray tube 10 has a faceplate which includes a phosphor
which produces an image within a narrow bandwidth of light centered at a wavelength
No which is around 453 nanometers. This is in the green portion of the spectrum for
visible light. While tilting of the faceplate of the cathode ray tube 10 does not
form a part of the invention it has been known that such tilting does improve the
performance of the optical system.
[0019] The relatively low intensity light of the display from the cathode ray tube 10 is
transmitted through the solar filter 12. In the preferred embodiment, this solar filter
12 is a plate of photochromic material of a type, for example, which will be described
in more detail with reference to Fig. 3. This solar filter 12 is preferably positioned
in the back focal plane of the relay optics 14.
[0020] The relay optics receive the light of the display information transmitted through
the solar filter. The relay optics 14 comprises a series of lenses which are configured
to partially compensate for aberrations in the holographic lens system. The light
of the display from the relay optics 14 is directed to a folding reflector 16.
[0021] The folding reflector 16 can be a mirror or prism which partially corrects the intermediate
image and serves to reduce the overall length of the optical system.
[0022] The intermediate image received via the folding reflector 16 is reflected to a combiner
18 positioned between a windscreen 20 and the eye of a viewer. This combiner 18 is
a holographic diffraction optics lens which reflects light that is within a narrow
bandwidth and is substantially transmissive to light of all other wavelengths according
to the well-known laws of Bragg diffraction. One method of fabricating this combiner
18 is explained in more detail in U.S. Patent No. 3,940,204 and will not be repeated
here.
[0023] In operation, the combiner 18 reflects the narrow bandwidth light from the display
to the exit pupil. Thus, the display image is in effect combined with and superimposed
on the scene in front of the vehicle, as viewed through a windscreen 20 by the viewer.
Since the combiner 18 is tuned to reflect light in the preferred bandwidth centered
at around l
b, this portion of the broad bandwidth light from the scene is removed.
[0024] Referring now to Fig. 2, when the sun is within certain critical angles, solar noise
in the form of the broad spectrum solar radiation is transmitted through the combiner
18 and travels back down through the optical system toward the face of the cathode
ray tube 10. However, the. solar filter 12 effectively blocks this solar radiation
thereby preventing it from washing out the display information on the face of the
cathode ray tube 10.
[0025] More specifically, the broad spectrum solar radiation shining on one face of the
combiner 18 has a narrow bandwidth notched out by the reflecting capabilities of the
combiner 18. As previously stated, the reflective capabilities of the combiner 18
are tuned to a bandwidth that includes the wavelength X
o. Thus, this portion of the solar radiation spectrum is reflected off of the face
of the combiner element 18 and the remainder is transmitted down through the optical
system to the folding reflector 16, along the optical axis through relay optics 14,
and to the solar filter 12. Of course, it is also possible that the incident angle
of solar radiation could be beyond the bounds of the combiner element 18 and still
shine back down the optical path.
[0026] The solar filter 12 is preferably positioned at the back focal plane of the relay
optics 14. Consequently, the solar energy is focused at a point or spot within the
body of the solar filter 12. As a result, the photochromic material of the solar filter
12 turns dark at the focal point thereby creating a darkened portion which effectively
blocks the solar energy. Consequently, the solar energy can not strike the face of
the cathode ray tube 10 where it could otherwise have washed out the display image.
[0027] Moreover, as the relative angle of the sun moves, the focal point for the solar energy
also moves. As a result, the darkened area moves through the solar filter 12 at the
reaction rate of the photochromic material and reversibly lightens and clears from
its previously darkened condition at the recovery rate of the photochromic material
when the focused solar radiation is removed from the spot. Consequently, there is
continued correction for the relative movement of the solar energy within the whole
range of critical solar angles.
[0028] Of course, an image of the darkened spot is focused upstream and can be seen through
the exit pupil as a spot in space which has a tendency to block the display information.
Since this spot has a very small diameter typically 0.203 mm or less (0.08") the binocular
vision of the viewer is such that he can usually see around the spot. Moreover, if
the image of the spot is at a critical viewing angle or if the "comet-tail" effect
created by the finite recovery time of the photochromic material is such that binocular
vision is not able to compensate for the slight blockage in viewing, the viewer merely
has to move his head to reposition his eyes laterally relative to the optical axis
thereby enabling him to see past the image of the darkened spot.
[0029] Fig. 3 is a detailed description illustrating a preferred embodiment of an optical
system constructed for a particular purpose. Of course, it should be understood that
modifications can be made in this configuration for other purposes and that the solar
filter 12 would be equally applicable.
[0030] As stated previously, the image display surface 10 is preferably the face of a cathode
ray tube which has a phosphor which produces a light image of a very narrow bandwidth
centered at a wavelength of about 543 nanometers. Of course, it should be understood
that the image could be produced by another type of display such as, for example,
a liquid crystal display projector or other displays which create light of sufficient
brightness and at the proper wavelength to be focused through the optical system and
diffracted to the exit pupil.
[0031] A housing 25 encloses the face of cathode ray tube 10, solar filter 12, relay optics
14, folding reflector 16 and has an opening which allows the rays of the intermediate
image from the folding reflector to be directed to combiner 18. Moreover, the housing
25 supports the enclosed optical elements.
[0032] The solar filter 12 is a flat plate of photochromic material which is substantially
transparent to the image of the display information in that the intensity of the narrow
bandwidth display information is significantly lower than the intensity of any broad
spectrum solar energy which is focused on a spot within the solar filter 12. While
the solar filter 12 is shown as a thin, flat plate, it must have sufficient thickness
and shape so that the solar radiation focused downstream through relay optics 14 is
concentrated on a small portion of the solar filter 12. Thus, the solar filter 12
is positioned so that the focal point of solar radiation is within the solar filter
material or at least very near to it. Thus, the back focal plane of the relay optics
is the preferred location for the solar filter 12.
[0033] The material of the solar filter 12 is preferably a photochromic glass which changes
transmittance reversibly, under the action of light. The host material in one particular
glass is silicon glass and the active materials are considered to be silver halide
crystals formed by the crystallization from the glassy matrix during initial cooling
or subsequent heat treatment of the glass. The amount of silver can be typically 0.5%
or less and the crystal size is small compared to a wavelength of light so that scattering
is not of concern. Photochromic glasses of this type are described in the article
by G. K. Megla, "Exploitation of Photochromic Glass", Optics and Laser Technology,
April 1974, pp 61-68.
[0034] With the particular material, the darkening time should be relatively fast in response
to the high intensity, broad spectrum, focused solar radiation directed to it. The
clearing rate should also be relatively fast in the area from which the focused light
is removed in order to reduce the "comet-tail" effect.
[0035] The relay optics 14 include a series of lenses disposed along the optical axis. One
set of lenses that could be used are those disclosed in U.S. Patent No. 3,940,204
relative to Fig. 16 thereof. Specifically, the relay lenses 14 include positive spherical
lens elements 30, 32, 36 and 40, a negative spherical lens element 34 and a negative
cylindrical lens element 38. The curvature of the negative cylindrical lens elements
are illustrated in the side view of Fig. 3, but it is within the scope of this invention
that the lens elements may be positive.
[0036] The cylindrical lens element 38 corrects the residual axial astigmatism in the combiner
18. It is to be noted that this lens element 38 can be replaced by a cylindrical surface
on one or more of the positive spherical lens elements in the relay optics 14. The
axial coma is best corrected in the illustrated arrangement by a decentration of the
negative spherical lens 34. It is to be noted that axial coma can be corrected by
decentering any appropriate lens in the relay optics 14.
[0037] The folding reflector 16 is a prism which provides a partial correction to axial
coma. However, this is not its main purpose. The prism 16 is a wedge with a mirror
on its rear surface. Rays of the image at the top of the intermediate image transverse
a thicker wedge of glass than those rays imaged at the bottom of the intermediate
image. This reduces considerably the tilt of both the intermediate image focal surface
and, hence, also reduces the tilt of the object surface. The final tilt of the object
surface in the illustrated configuration can be about 15° but would otherwise have
been over 30° if a prism were not used. The rays of the intermediate image are directed
from this prism 16 to the combiner 18.
[0038] The combiner 18 can have its diffraction grating pattern constructed, for example,
by means of the holographic optics disclosed in U.S. Patent No. 3,940,204. Structurally,
the combiner 18 includes a layer 50 of photosensitive organic material such as dichromated
gelatin or photographic emulsion which has the diffraction grating pattern recorded
on it. A more detailed discussion of such materials and holographic recording can
be found in a book by Robert J. Collier et al., Optical Holography, (New York: Academic
Press, 1971), · p. 293 et seq. This thin layer 50 is sandwiched between two layers
of glass 52 and 54 which provide structural strength and protect it from physical
damage. The overall combiner 18 has a generally spherical or concave configuration
such that the intermediate image of the display information is reflected from the
concave surface toward the exit pupil.
[0039] Functionally, the diffraction grating pattern will diffract and reflect light with
a high optical efficiency in a narrow band in accordance with the well-known principles
of Bragg diffraction. However, the combiner 18 exhibits a high transmittance of light
outside of this narrow band of reflected light thus permitting the pilot to view the
outside scene at the same time he is viewing the reflected display information. Consequently,
the display information appears to be combined with and superimposed on the scene
being viewed. Moreover, the spherical shape of the combiner 18 is such that it balances
with the other optical elements of the optical system such that both axial coma and
axial astigmatism are corrected in the image of the display information reflected
to the exit pupil.
[0040] While the combiner 18 has been described as a reflective type diffraction pattern,
it is possible to use a transmissive type which redirects the light waves in the manner
exemplified in U.S. Patent No. 3,915,548 issued to E. F. Opittek et al on October
28, 1975.
[0041] As previously stated, by placing the solar filter 12 between the faceplate of the
cathode ray tube 10 and the relay lenses 14, it is possible to shorten the length
of the optical axis thereby making the optical system compact. As a result, it is
possible to keep the optical system out of the path of the ejection line as shown
in the dashed-lines in Fig. 3. Thus, some of the space constraints associated with
cramped cockpits are avoided.
[0042] While the solar filter 12 has been described as photochromic type of material, it
could be constructed of other types of material. For example, it could be made of
a type of sensor device such as a transmission type liquid crystal photodetector matrix
either of the absorbing or reflecting kind. If reflective, the direction of reflection
is oriented out of the normal optical path. Examples of liquid crystal devices of
the transmission type are disclosed in U.S. Patent No. 3,744,879, issued on July 10,
1973 to T. D Beard et al and U.S. Patent No. 3,811,180 issued on May 21, 1974 to M.
Braunstein.
[0043] As illustrated in Fig. 4, the solar filter 12 can be constructed in a different configuration
other than a flat plate. Here the solar filter 12 is constructed as an optical prism
of photochromic material. Thus, as solar energy focused downstream through the relay
optics 14 is focused and concentrated as a spot in the optical prism 56, it causes
a small volume of material within the prism to darken thereby blocking solar radiation
and preventing it from impinging on the image surface 10 containing the display information
and thereby washing out or otherwise affecting the image.
[0044] Although the present invention has been shown and described with reference to particular
embodiments, nevertheless various changes and modifications which are obvious to a
person skilled in the art to which the invention pertains are deemed to lie within
the spirit, scope and contemplation of the invention.
1. Ein Filter für sichtbares Licht für eine projezierte Frontscheibenanzeigeeinrichtung,
die Anzeigevorrichtungen zur Erzeugung eines Abbildes von anzuzeigenden Informationen
und ein optisches System mit Linsenvorrichtungen zur Führung des Abbildes des anzuzeigenden
Informationen in das Auge eines Beobachters aufweist, gekennzeichnet durch:
optische Filtervorrichtungen (12), die zwischen Anzeigevorrichtungen (10) und Linsenvorrichtungen
(14) angeordnet sind, die umkehrbar reziprok sind, um das durch die Linsenvorrichtungen
(14) fallende sichtbare Licht zurück zur Anzeigevorrichtung (10) zu fokussieren, um
das fokussierte sichtbare Licht abzublokken, und die für die anzuzeigenden Informationen
durchlässig sind, mit Ausnahme der Stellen, an denen das fokussierte sichtbare Licht
abgeblockt wird.
2. Vorrichtung nach Anspruch 1, dadurch gekennzeichnet, daß die optische Filtervorrichtung
(12) in der Brennebene für die rückwärtige Brennweite der Linsenvorrichtungen (14)
angeordnet ist.
3. Vorrichtung nach Anspruch 1, dadurch gekennzeichnet, daß die optische Filtervorrichtung
(12) in etwa in der Brennebene für die rückwärtige Brennweite der Linsenvorrichtung
(14) angeordnet ist.
4. Vorrichtung nach einem der Ansprüche 1, 2 oder 3, dadurch gekennzeichnet, daß die
optische Filtervorrichtung (12) aus fotochromatischem Material hergestellt ist.
5. Vorrichtung nach einem der Ansprüche 1, 2 oder 3, dadurch gekennzeichnet, daß die
optische Filtervorrichtung (12) eine Platte aus fotochromatischem darstellt.
6. Vorrichtung nach einem der Ansprüche 1, 2 oder 3, dadurch gekennzeichnet, daß die
optische Filtervorrichtung (12) ein prismatisches optisches Element (12') darstellt,
das aus fotochromatischem Material hergestellt ist.
7. Vorrichtung nach einem der Ansprüche 1, 2 oder3, dadurch gekennzeichnet, daß die
optische Filtervorrichtung (12) eine Flüssigkristall-Fotodetektor-Matrix vom Transmissionstyp
ist.
1. Filtre de lumière visible dans un affichage tête haute comprenant un moyen d'affichage
pour produire une image d'informations d'affichage et un système optique comprenant
un moyen d'objectif pour diriger l'image des informations d'affichage vers l'oeil
d'un observateur, caractérisé par:
un moyen de filtre optique (12) disposé entre le moyen d'affichage (10) et le moyen
d'objectif (14) qui répond de façon réversible à de la lumière visible focalisée dirigée
à traverse le moyen d'objectif (14) vers l'arrière vers le moyen d'affichage (10)
pour bloquer la lumière visible focalisée et qui est transmissif pour les informations
d'affichage sauf à l'emplacement localisé où de la lumière visible focalisée es' bloquée.
2. Dispositif selon la revendication 1, dans lequel le moyen de filtre optique (12)
est situé dans le plan focal correspondant à la distance focale arrière du moyen d'objectif
(14).
3. Dispositif selon la revendication 1, dans lequel le moyen de filtre optique (12)
est disposé sensiblement au niveau du plan focal correspondant à la distance focale
arrière du moyen d'objectif (14).
4. Dispositif selon les revendications 1, 2 ou 3, dans lequel le moyen de filtre optique
(12) est constitué d'une substance photochrome.
5. Dispositif selon l'une des revendications 1, 2 ou 3, dans lequel le moyen de filtre
optique (12) est une plaque de substance photochrome.
6. Dispositif selon l'une des revendications 1, 2 ou 3, dans lequel le moyen de filtre
optique (12) est un élément optique prismatique (12') constitué d'une substance photochrome.
7. Dispositif selon l'une des revendications 1, 2 ou 3, dans lequel le moyen de filtre
optique (12) est une matrice de photodétecteurs à cristaux liquides du type à transmission.